Almost impossible to accurately measure from what I’ve read. There rated by the manufacturer. The number really doesn’t mean much. It’s just relative to weaker and stronger numbered springs from the same manufacturers. I believe most handguns come stock with recoil springs that can handle a +P load. A rough goal is to have about 6-8’ of ejection distance for the ammo load. Just a rough guideline but 2’ and the slides moving to slow. 8+ feet and the slides moving to fast.
Recoil spring is also completely independent from the main/hammer spring. They don’t affect each other.
